Chiller systems for applications in commercial or industrial building HVAC systems typically include relatively large electric motors for powering a compressor. The motors may range in horsepower from 100 HP up to 5,000 HP or larger. Many of these systems include variable speed drives (VSD) for controlling the speed of the motor in response to cooling demand. Motors and VSDs of this size must be designed for the applicable main distribution voltages. In the case where low voltage mains (600 volts or less) are supplying the chiller system, higher current capacity is required. The relatively high current load requires larger more expensive cables, step-down transformers, and switchgear.
Conversely, where the voltage main supplies medium/high voltage (greater than 600 volts) to the chiller system, the current capacity requirements are low relative to the low voltage system requirements. However, other considerations such as equipment costs, complexity and safety must also be taken into account. Medium/high voltage (MV) switchgear, typically 4,160V, requires sophisticated arc suppression, insulation, and safety characteristics. Moreover, because of the greater potential danger of electrocution, only specially trained and qualified maintenance personnel may be permitted to perform the highly specialized maintenance operations of the medium/high voltage distribution system. Corona, flashover and arcing are some of the adverse factors associated with medium/high voltage distribution systems. The presence of moisture and dust particles further contributes to the adverse factors, resulting in increased switchgear costs. Additionally, power semiconductors are not generally available for medium/high voltage applications, and require especially high reverse-voltage characteristics, further adding to the increased cost size and complexity of such equipment.
An electric system suitable for a typical low voltage chiller system, due to high current requirements, can include large expensive conductors, switchgear and transformers for a low voltage distribution system. Alternatively, due to the lower current requirements, the components of the medium/high voltage switchgear could be smaller; however, due to the higher voltage significantly greater equipment clearances are required. As a result, there is no space saving advantage associated with the medium/high voltage switchgear and drives.
Floor mounted medium/high voltage drives and starters are commercially available. One such motor drive is a model T300 MVi manufactured by the Toshiba International Corporation. The T300 MVi variable speed drive arrangement includes a rather complex 24 pulse input transformer having 12 three-phase secondary windings to supply the drive input rectifiers, plus two secondary windings for two control voltage levels. The floor area required to accommodate most medium/high voltage VSDs, including the T300MVi, is approximately 50 ft.2 to 100 ft.2, which in many instances is equal to or greater than the floor area required for the chiller. Due to the limited floor area allocated for HVAC systems in buildings, it is desirable to minimize the floor area utilized by the chiller system, while maintaining an appropriately sized chiller system.
U.S. Pat. No. 5,625,545 discloses an electric drive apparatus and method for controlling medium/high-voltage alternating current motors wherein a multi-phase power transformer supplies multi-phase power to multiple power cells. In one embodiment of the '545 patent which can be applied to 2300 VAC inductive motor loads, three power cells are used for each of the three phase output lines. In another embodiment, which may be applied to a 4160 VAC inductive motor load, five power cells may be used for each of the three phase output lines. Such an embodiment can have eleven voltage states. Such multiple power cell transformers are more costly and consume an excessive amount of space to accommodate the large number of cells and clearances space required for medium/high voltage motors. As a result they require separate mounting cabinets and cannot be mounted directly on the chiller equipment to save floor space.
Therefore, there is a need for a low voltage chiller system that can be connected to a medium/high voltage AC power source with a step-down transformer that is sufficiently compact to permit the transformer to be mounted directly on the chiller, integral with the VSD.